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        <title>Document</title>
        <script>
            /* 
                普通赋值运算符: =
                    功能：将=号右边的值，赋值给=号左边的变量。
                复合赋值运算符：
                    x=   += -=
             */
            /*  var num = 10 + 20;
             num = num + 10;
             alert(num); */

             var num = 10;
            //  num = num + 5;
             num += 5;  //上述表达式的简写
             alert(num);
        </script>
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